Ldmos transistor with asymmetric spacer as gate

ABSTRACT

The present invention provides a laterally diffused metal oxide semiconductor (LDMOS) transistor and a method for fabricating it. The LDMOS transistor includes an n-type epitaxial layer formed on a p-type substrate, and an asymmetric conductive spacer which acts as its gate. The LDMOS transistor also includes a source and a drain region on either side of the asymmetric conductive spacer, and a channel region formed by ion-implantation on the asymmetric conductive spacer. The height of the asymmetric conductive spacer increases from the source region to the drain region. The channel region is essentially completely under the asymmetric conductive spacer and has smaller length than that of the channel region of the prior art LDMOS transistors. The LDMOS transistor of the present invention also includes a field oxide layer surrounding the active region of the transistor, and a thin dielectric layer isolating the asymmetric conductive spacer from the n-type epitaxial layer.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/592,011, entitled LDMOS TRANSISTOR WITH ASYMMETRIC SPACER ASGATE filed Nov. 18, 2009 which is incorporated herein by reference forall purposes.

FIELD OF THE INVENTION

The present invention relates, in general, to a laterally diffused metaloxide semiconductor (LDMOS) transistor and, more specifically, to anLDMOS transistor having an asymmetrical conductive spacer acting as agate.

BACKGROUND

An LDMOS transistor conventionally includes an electrically conductivegate enclosed between two electrically insulating spacers, a sourceregion, a drain region, a channel region, and a drift region. A positivepotential is applied to the gate, which causes the flow of electronsfrom the source region to the drain region through the channel region ofthe LDMOS transistor. Due to the insulating nature of the spacers, theydo not act as part of the gate, and the gate voltage can be applied onlythrough the gate and not through the spacers.

A conventional LDMOS transistor 100, as described above, is illustratedin FIG. 1. LDMOS transistor 100 includes an n-type buried layer 104formed on a p-type substrate 102. An n-type epitaxial layer 106 is grownon n-type buried layer 104, and field oxide layers 108 a and 108 b areformed on n-type epitaxial layer 106 to define the active region ofLDMOS transistor 100. Typically, the active region of LDMOS transistor100 is the region on n-type epitaxial layer 106 where LDMOS transistor100 is being fabricated or formed.

LDMOS transistor 100 also includes a p-well 110 in which a source region112 is formed. P-well 110 can be formed through ion implantation ordiffusion of any p-type element such as boron. Similarly, the sourceregion 112 can also be formed through ion implantation or diffusion ofany n-type element such as arsenic. Similar arsenic implantation can beused to form a drain region 114 of LDMOS transistor 100.

Further, LDMOS transistor 100 includes a gate 116, for example, apolysilicon gate that is partially over n-type epitaxial layer 106 andpartially over p-well 110. As shown in FIG. 1, gate 116 is isolated fromn-type epitaxial layer 106 and p-well 110 by a thin dielectric layer118, which can be, for example, a thin silicon oxide (SiO2) layer.Further, on the sidewalls of gate 116, spacers 120 a and 120 b areformed. These spacers are non-conductive in nature and can be formed byusing dielectric material such as silicon oxide (SiO₂) or nitride. Thoseordinarily skilled in the art will appreciate that the region under thespacers is lightly doped N-region, commonly known as NLDD (n-typelightly doped diffusion), but is not shown for simplicity.

Typically, whenever a preset positive gate voltage is applied at gate116, electrons (minority carriers) present in p-well 110 are attractedtoward gate 116, and consequently a channel region 122 is formed.Channel region 122 connects source region 112 to drift region 124 ofLDMOS transistor 100. When drain-to-source voltage (not shown in FIG. 1)is applied to LDMOS transistor 100, the electrons present in sourceregion 112 travel through channel region 122 and drift region 124 todrain region 114, thus enabling the flow of electrons from the source tothe drain in LDMOS transistor 100.

Conventional LDMOS transistor 100, as described above, has thelimitation of high parasitic capacitance and channel resistance. Theparasitic capacitance of LDMOS transistor 100 is due to the “capacitor”formed between gate 116 and channel region 122. The value of theparasitic capacitance is directly related to the product of the width(not shown in FIG. 1) and the length of channel region 122. Further, thechannel resistance of LDMOS transistor 100 is due to the resistanceoffered by channel region 122, and its value is also related to thelength and the width of channel region 122.

The high parasitic capacitance and channel resistance of LDMOStransistor 100 causes the RC constant of LDMOS transistor 100 toincrease, and hence the time required for charging and discharging ofparasitic gate capacitor of LDMOS transistor 100 also increases. Thishampers the performance of LDMOS transistor 100, and the speed of thecircuit which utilizes it also decreases. Therefore, continuous effortsare being made to reduce the parasitic capacitance and channelresistance of LDMOS transistor 100.

To overcome the above-mentioned problems, the present invention providesan LDMOS transistor that has much lower channel resistance and parasiticcapacitance than that of the prior art LDMOS transistors. A method tofabricate the said LDMOS transistor is also provided.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method forfabricating an LDMOS transistor is provided. The method includes forminga semiconductor layer of a first conductivity type on a semiconductorsubstrate. The semiconductor layer is, for example, an epitaxial layerand the first conductivity type is n-type conductivity. In accordancewith an embodiment of the present invention, the n-type epitaxial layeris separated from the semiconductor substrate by an n-type buried layer.

The method further includes forming a dielectric layer on thesemiconductor layer. The dielectric layer can be, for example, a thinsilicon oxide (SiO₂) layer formed on the semiconductor layer. The methodalso includes forming an asymmetrical conductive spacer on thesemiconductor layer. The asymmetrical conductive spacer acts as a gateof the LDMOS transistor and is insulated from the semiconductor layer bythe dielectric layer. In accordance with an embodiment of the presentinvention, the asymmetrical conductive spacer divides the semiconductorlayer into two regions, a first region and a second region, and theshape of the asymmetrical conductive spacer is such that the height ofthe asymmetrical conductive spacer increases from the first region tothe second region. Furthermore, the method includes etching thedielectric layer to remove portions of the dielectric layer from thefirst region and the second region.

The method also includes forming a field oxide layer on thesemiconductor layer and then etching the field oxide layer to define theactive region of the LDMOS transistor. In other words, the field oxidelayer is etched in such a way that it surrounds the area of thesemiconductor layer where the LDMOS transistor is being fabricated.Those ordinarily skilled in the art will understand that the process offorming the field oxide layer to define the active region of the LDMOStransistor is well known in the art. Also, the field oxide layer can beformed before the formation of the dielectric layer and the asymmetricalconductive spacer, without departing from of the scope of the presentinvention.

Once the active region of the LDMOS transistor is defined, a firstimplantation is performed on the first region of the semiconductor layerby using a first type of dopant of a second conductivity. The firstimplantation is done to form a well of the second conductivity type inthe first region of the semiconductor layer. In accordance with anembodiment of the present invention, the second conductivity type isp-type conductivity and the first type of dopant is boron. Also, thefirst implantation is performed by using the first energy ofimplantation, which can be for example 50 kv, and a typical dosageimplant is in the range of 10¹²-10¹³/cm³. The method includes performinga second implantation by using a second type of dopant of the firstconductivity type to form a source region and a drain region of theLDMOS transistor. The second type of dopant can be, for example, any oneof arsenic or phosphorous or both. The source region is formed such thatit is partially in the well of the second conductivity type andpartially underneath the asymmetric conductive spacer.

Furthermore, the method includes performing a third implantation byusing a third type of dopant of the second conductivity type on theasymmetric conductive spacer to form a channel region of the LDMOStransistor. The channel region is formed in the semiconductor layer andis completely under the asymmetric conductive spacer. In accordance withan embodiment of the present invention, the third type of dopant isboron, and the third implantation is performed using the second energyof implantation, which can be for example 50 kv, and the dosage implantcan be, for example in the range of 10¹³-10¹⁴/cm³. In anotherembodiment, this region can be formed by a combination of boron andphosphorous with both in the same range of doping concentration.

According to another embodiment of the present invention, an LDMOStransistor is provided. The LDMOS transistor includes a semiconductorlayer of a first conductivity type formed on a semiconductor substrate.Further, the LDMOS transistor includes an asymmetric conductive spacerformed on the semiconductor layer which acts as its gate and isinsulated from the semiconductor layer by a thin dielectric layer. Inaccordance with an embodiment of the present invention, the asymmetricconductive spacer divides the semiconductor layer into two regions, afirst region and a second region; and the height of the asymmetricconductive spacer increases from the first region to the second region.

Further, the LDMOS transistor includes a well of a second conductivitytype and a source region of the first conductivity type, which ispartially in the well and partially underneath the asymmetric conductivespacer. The source region is formed in the first region of thesemiconductor layer.

The LDMOS transistor further includes a drain region which is formed inthe second region of the semiconductor layer, and a channel region whichis formed in the semiconductor layer. The channel region is completelyunder the asymmetric conductive spacer. In accordance with an embodimentof the present invention, the channel region and the drain region areseparated by a drift region of the first conductivity type. The LDMOStransistor also includes a field oxide layer surrounding the activeregion of the LDMOS transistor.

According to yet another embodiment of the present invention, a powerfield effect transistor (FET) is provided. The power FET includes asemiconductor layer of a first conductivity type formed on asemiconductor substrate. As already mentioned, the semiconductor layeris an epitaxial layer and the first conductivity type is n-type. Thepower FET further includes a plurality of asymmetric conductive spacersformed on the semiconductor layer. The plurality of asymmetricconductive spacers act as a gate of the power FET and are insulated fromthe semiconductor layer by a thin dielectric layer. Further, the heightof each asymmetric conductive spacer increases from source region todrain region of the power FET.

Further, the power FET includes a plurality of wells of a secondconductivity type formed in the semiconductor layer and a plurality ofsource regions of the power FET of the first conductivity type. Eachsource region of the power FET is partially in the well of the secondconductivity type and partially underneath one or more asymmetricconductive spacers of the plurality of asymmetric conductive spacers.The power FET also includes a plurality of drain regions of the firstconductivity type formed in the semiconductor layer and a plurality ofchannel regions of the second conductivity type formed in thesemiconductor layer corresponding to the plurality of asymmetricconductive spacers. Each channel region of the plurality of channelregions is completely under the asymmetric conductive spacers of thepower FET.

In accordance with an embodiment of the present invention, theasymmetric conductive spacers are connected in pairs. Each pair of theasymmetric conductive spacers forms a frame structure outside the activeregion of the power FET. Also, the pairs of asymmetric conductivespacers are connected to each other through a conductive materialoutside the active region of the power FET. The conductive material canbe, for example, poly silicon.

Apart from the components mentioned above, the power FET also includes afield oxide layer surrounding the active area of the power FET and theplurality of drift regions separating the plurality of drain regions andthe plurality of channel regions.

One objective of the present invention is to provide an LDMOS transistorwith lower parasitic capacitance and channel resistance than those ofthe prior art LDMOS transistors. To this end, the length of theeffective channel region of the LDMOS transistor is reduced byperforming ion-implantation on an asymmetric conductive spacer acting asa gate.

Another objective of the present invention is to provide a method tofabricate an LDMOS transistor, which has lower parasitic capacitance andchannel resistance than those of the prior art LDMOS transistors.

Yet another objective of the present invention is to provide an LDMOStransistor in which the entire channel region is underneath theasymmetric conductive spacer (which acts as a gate in the presentinvention) of the LDMOS transistor.

Another objective of the present invention is to provide a power FETthat has a frame gate structure, with asymmetric conductive spacersacting as its gate. The frame gate structure reduces the width of thegate, thereby decreasing the parasitic capacitance and channelresistance of the power FET.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention will hereinafter be describedin conjunction with the appended drawings provided to illustrate and notto limit the invention, wherein like designations denote like elements,and in which:

FIG. 1 illustrates a cross-sectional view of a prior art LDMOStransistor;

FIG. 2 illustrates a cross-sectional view of an LDMOS transistor, inaccordance with an embodiment of the present invention;

FIG. 3 is a flowchart illustrating a method for fabricating the LDMOStransistor, in accordance with an embodiment of the present invention;

FIG. 4 is a cross-sectional view of a semiconductor structureillustrating an epitaxial layer grown over a semiconductor substrate, inaccordance with an embodiment of the present invention;

FIG. 5 is a cross-sectional view of the semiconductor structureillustrating a thin dielectric layer formed on the epitaxial layer, inaccordance with an embodiment of the present invention;

FIG. 6 is a cross-sectional view of the semiconductor structureillustrating an asymmetric conductive spacer formed on the thindielectric layer, in accordance with an embodiment of the presentinvention;

FIG. 7 is a cross-sectional view of the semiconductor structureillustrating the etched dielectric layer, in accordance with anembodiment of the present invention;

FIG. 8 is a cross-sectional view of the semiconductor structureillustrating a field oxide layer defining the active region of the LDMOStransistor, in accordance with an embodiment of the present invention;

FIG. 9 is a cross-sectional view of the semiconductor structureillustrating the formation of a p-well by using a first ionimplantation, in accordance with an embodiment of the present invention;

FIG. 10 is a cross-sectional view of the semiconductor structureillustrating the formation of the source region and the drain region ofthe LDMOS transistor by using a second ion implantation, in accordancewith an embodiment of the present invention;

FIG. 11 is a cross-sectional view of the semiconductor structureillustrating the formation of the channel region of the LDMOS transistorby using a third ion implantation, in accordance with an embodiment ofthe present invention;

FIG. 12 illustrates a top view of a power FET with frame gate structure,in accordance with an embodiment of the present invention; and

FIG. 13 illustrates a cross-sectional view of the power FET taken alongline A-A in FIG. 12, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates a cross-sectional view of an LDMOS transistor 200, inaccordance with an embodiment of the present invention. LDMOS transistor200 includes an n-type epitaxial layer 206 formed over a p-typesubstrate 202. N-type epitaxial layer 206 is separated from p-typesubstrate 202 by an n-type buried layer 204.

The active region of LDMOS transistor 200 is surrounded by a field oxide(FOX) layer 208 a, 208 b, which is generally formed to isolate LDMOStransistor 200 from other devices (not shown) formed on p-type substrate202. Essentially, field oxide layer 208 a, 208 b defines the area whereLDMOS transistor 200 is fabricated.

LDMOS transistor 200 further includes a p-well 210 formed in n-typeepitaxial layer 206 and a source region 212 formed partially in p-well210. P-well 210 can be formed by using any p-type dopant such as boron,and source region 212 can be formed by using an n-type dopant such asarsenic. Further, LDMOS transistor 200 includes an asymmetric conductivespacer 214, which is isolated from n-type epitaxial layer 206 by a thindielectric layer 216.

In accordance with an embodiment of the invention, the height ofasymmetric conductive spacer 214 is thinner toward source region 212, asshown in FIG. 2. The shape of asymmetric conductive spacer 214 isdesigned to facilitate the formation of a channel region 218 that is ofmuch smaller length than the prior art LDMOS transistors. Channel region218 is formed when ion-implantation is performed on asymmetricconductive spacer 214. According to an embodiment of the presentinvention, the entire channel region 218 is underneath asymmetricconductive spacer 214. The process of formation of channel region 218and all steps involved in fabricating LDMOS transistor 200 will bedescribed in conjunction with FIG. 3.

LDMOS transistor 200 further includes a drain region 220 having n-typeconductivity and typically having the same doping level as that ofsource region 212.

The operation of LDMOS transistor 200 will now be described briefly,assuming that a positive drain-to-source voltage and gate voltage isapplied to LDMOS transistor 200. In the present invention, asymmetricconductive spacer 214 acts as a gate of LDMOS transistor 200, and thegate voltage is applied to asymmetric conductive spacer 214 through gatecontacts (not shown). When a preset positive voltage is applied toasymmetric conductive spacer 214, electrons from source region 212 movethrough channel region 218 to drain region 220. The region betweenchannel region 218 and drain region 220 is called a drift region, andelectrons pass through this region with the help of “drift” attained bythem due to the potential difference between the drain and the sourceregions.

The process of fabricating LDMOS transistor 200 in accordance with oneembodiment will now be described in conjunction with FIGS. 3-11.

FIG. 3 is a flowchart illustrating a method for fabricating LDMOStransistor 200, in accordance with an embodiment of the presentinvention. While describing FIG. 3, references will be made to FIGS.4-11 to illustrate different steps in the formation of LDMOS transistor200.

At step 302, a semiconductor layer of n-type conductivity is formed overa p-type semiconductor substrate. As already explained in conjunctionwith the previous figure and as shown in FIG. 4, the said semiconductorlayer is n-type epitaxial layer 206 formed over p-type substrate 202. Inaccordance with an embodiment of the present invention, n-type epitaxiallayer 206 is separated from p-type substrate 202 by n-type buried layer204.

At step 304, dielectric layer 216 is formed on n-type epitaxial layer206, as shown in FIG. 5. Typically, dielectric layer 216 is a thinsilicon oxide (SiO₂) layer. At step 306, asymmetric conductive spacer214 is formed over n-type epitaxial layer 206 in such a way that it isseparated from it by dielectric layer 216 (as shown in FIG. 6).

In accordance with an embodiment of the present invention, asymmetricconductive spacer 214 is formed by using a lift-off process, i.e., byusing a sacrificial layer of oxide or nitride, which is removed afterthe formation of asymmetric conductive spacer 214. The process of usingsacrificial materials to create structures on semiconductor substratesis well known in the art and will not be described here. Thoseordinarily skilled in the art can appreciate that asymmetric conductivespacer 214 can also be formed by using a simple etching process, insteadof the lift-off process as mentioned above, without departing from thescope of the invention.

As shown in FIG. 6, asymmetric conductive spacer 214 divides n-typeepitaxial layer 206 into two regions, a first region and a secondregion, and at step 308, dielectric layer 216 is etched from the firstregion and the second region so that it remains only underneathasymmetric conductive spacer 214 (as shown in FIG. 7).

At step 310, a field oxide layer is formed on n-type epitaxial layer206, and at step 312, the field oxide layer is etched to define theactive region of LDMOS transistor 200. This is shown in FIG. 8, wherethe etched field oxide layer is depicted as field oxide layer 208 a, 208b. A typical process to prepare field oxide layer 208 a, 208 b is tofirst form a thick oxide layer on n-type epitaxial layer 206 (includingasymmetric conductive spacer 214) and then etch the oxide layer fromthose regions where LDMOS transistor 200 is to be formed. In this way,field oxide layer 208 a, 208 b will surround the area of n-typeepitaxial layer 206 where LDMOS transistor 200 is formed, therebyisolating it from other devices formed on the same p-type substrate 202.

At step 314, a first implantation is performed to form p-well 210 in thefirst region of n-type epitaxial layer 206. In accordance with anembodiment of the present invention, the energy of the firstimplantation is 50 kv and the dosage implant is in the range of 10¹² to10¹³/cm³. Typically, the dopant used for the implantation is boron.Typically, the first implantation is performed by using a mask 902,which masks all the regions on n-type epitaxial layer 206, except theregion where p-well 210 is being formed (as shown in FIG. 9).

At step 316, a second implantation is performed to form source region212 and drain region 220 of LDMOS transistor 200. FIG. 10 shows theprocess of implantation done to form source region 212 and drain region220. Typically, the energy of the implantation is 50 kv. Dopant arsenicor phosphorous or both is used in the n-type second implantation to formthe source and drain region. As shown in FIG. 10, source region 212 isformed in such a way that it is partially underneath asymmetricconductive spacer 214 due to the slope of the spacer and partially inp-well 210.

At step 318, a third implantation is performed on asymmetric conductivespacer 214 to form channel region 218 of LDMOS transistor 200. Asdepicted in FIG. 11 and according to one embodiment of the presentinvention, the third implantation is a halo (i.e., slant)ion-implantation and the energy of the implantation is 50 kv. The dopantused for the third implantation is similar to that used for the firstimplantation, i.e., boron. However, those ordinarily skilled in the artcan appreciate that channel region 218 can also be formed by using anyother p-type dopant such as gallium or indium. In accordance with anembodiment of the present invention, the halo ion-implantation isperformed after the formation of an oxide layer 1102 a, 1102 b over thesource and drain region of LDMOS transistor 200. Oxide layer 1102 a,1102 b “masks” the source and drain region of LDMOS transistor 200 andtherefore the ions used for the third implantation do not penetrate intothese regions. However, the ions penetrate the sloped portion of thespacer 214 in order to form the channel region 218. Essentially, theformation of oxide layer 1102 a, 1102 b eliminates the need forproviding a separate mask while channel region 218 is being formed.After channel region 218 is formed, oxide layer 1102 a, 1102 b is etchedresulting in the LDMOS transistor shown in FIG. 2.

Those ordinarily skilled in the art will appreciate that channel region218 can also be formed by using normal ion-impanation (rather thanhalo-ion implantation). In this case, the thickness of oxide layer 1102a, 1102 b is chosen such that ions penetrate through the thinner regionof asymmetric conductive spacer 214, but do not penetrate the source anddrain regions through oxide layer 1102 a, 1102 b.

Since the height of asymmetric conductive spacer 214 is thinner towardsource region 212, the shape of the formed channel region 218 is suchthat it is thicker toward p-well 210 and its depth decreases sharplyaway from p-well 210. This is due to the fact that during the process ofimplantation, the ions penetrate more deeply toward the thinner edge ofasymmetric conductive spacer 214, and hence, the depth of channel region218 is deeper toward p-well 210. As the height of asymmetric conductivespacer 214 increases from source region 212 to drain region 220, thedepth of channel region 218 decreases away from source region 212 (andp-well 210) as the ion penetration decreases. Further, due to the factthat ion implantation is performed on asymmetric conductive spacer 214,the formed channel region 218 is essentially completely underneathasymmetric conductive spacer 214 (which is the gate of LDMOS transistor200).

FIGS. 12 and 13 illustrate, respectively, a top view and across-sectional view taken along line A-A′, of a power FET 1200 withframe gate structure, in accordance with an embodiment of the presentinvention. Power FET 1200 includes an n-type epitaxial layer 1202 formedover a p-type substrate 1302. In accordance with an embodiment of theinvention, n-type epitaxial layer 1202 is separated from p-typesubstrate 1302 by an n-type buried layer 1304, which is more heavilydoped than n-type epitaxial layer 1202.

Power FET 1200 further includes a plurality of source regions 1204 and aplurality of drain regions 1206. Source regions 1204 and drain regions1206 are of n-type conductivity and are formed by using the same orsimilar fabrication process as described in conjunction with FIG. 3 forLDMOS transistor 200. Power FET 1200 also includes a plurality ofp-wells 1306 that are formed in n-type epitaxial layer 1202. Each p-wellof the plurality of p-wells 1306 in power FET 1200 is similar to p-well210 of LDMOS transistor 200 and is formed by using the same or similarfabrication process. Similar to LDMOS transistor 200, each source regionof power FET 1200 is partially in p-wells 1306 (as shown in FIG. 13),and partially underneath the one or more asymmetrical conductive spacersof a plurality of asymmetrical conductive spacers 1208. The plurality ofasymmetrical conductive spacers 1208 act as a gate of power FET 1200 andare insulated from type epitaxial layer 1202 by a dielectric layer 1308,typically made of SiO₂. Similar to asymmetrical conductive spacer 214 ofLDMOS transistor 200, the height of each of the plurality ofasymmetrical conductive spacers 1208 of power FET 1200 increases from asource region to a drain region.

In accordance with an embodiment of the present invention, the pluralityof asymmetrical conductive spacers 1208 are connected in pairs in a row(as shown in FIG. 12) and each pair forms a frame structure 1210 outsideactive region of power FET 1200. The plurality of asymmetricalconductive spacers 1208 are connected in pairs in a row to reduce theeffective width “W” of the channel of power FET 1200, thereby reducingparasitic capacitance and channel resistance of power FET 1200. Further,each pair of the asymmetrical conductive spacers is connected to otherpairs through a conductive material 1214 outside the structure of thepower FET, i.e., outside n-type epitaxial layer 1202.

Those ordinarily skilled in the art will appreciate that when a gate isconnected in a frame structure or “folded”, the effective width of thechannel gets divided by the number of folds. For example, in theembodiment shown in FIG. 12 the effective channel width is one fourth ofwhat it would have been if the gates were not folded or connected in theframe structure.

Conductive material 1214 can be made of, for example, polysilicon or anymetal such as WSi_(x). Since effectively all the asymmetrical conductivespacers of power FET 1200 are connected to each other through conductivematerial 1214, a gate contact (not shown) of power FET 1200 can beprovided on conductive material 1214.

Power FET 1200 also includes a plurality of channel regions 1310 formedunderneath the plurality of asymmetrical conductive spacers 1208.Similar to channel region 218 of LDMOS transistor 200, each channelregion of the plurality of channel region 1310 is of p-type conductivityand is essentially completely under an asymmetrical conductive spacer ofpower FET 1200. As already described in conjunction with FIG. 3, thechannel region underneath the asymmetrical conductive spacer is formedby performing ion-implantation on the asymmetrical conductive spacer,and the dopant used for implantation is similar to that used in theformation of a p-well.

Apart from the components described above, power FET 1200 also includesfield oxide layers 1312 a and 1312 b that surround its active region,and a plurality of drift regions (not shown), formed between theplurality of channel regions 1310 and the plurality of drain regions1206 when a positive gate voltage is applied on conductive material1214.

Various embodiments of the present invention provide several advantages.The length “L” of the channel region of LDMOS transistor, according toan embodiment of the present invention, is much shorter than the lengthof the channel region of the prior art LDMOS transistors. This leads tolower channel resistance and parasitic capacitance. Further, thepreferred embodiment of the present invention involves forming a “framegate structure” of asymmetric conductive spacers (acting as the gate) ofa power FET. This results in reduced width “W” of the channel and hencefurther decreases the channel resistance and parasitic capacitance.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not limited tothese embodiments only. Numerous modifications, changes, variations,substitutions and equivalents will be apparent to those skilled in theart without departing from the spirit and scope of the invention asdescribed in the claims.

What is claimed is:
 1. A method for fabricating a laterally diffusedmetal oxide semiconductor (LDMOS) transistor, the method comprising:forming a semiconductor layer of a first conductivity type over asemiconductor substrate; forming an asymmetric conductive spacer overthe semiconductor layer, the asymmetric conductive spacer acting as agate of the LDMOS transistor and being insulated from the semiconductorlayer by a dielectric layer, wherein the asymmetric conductive spacerdivides the semiconductor layer into a first region and a second region,and wherein a height of the asymmetric conductive spacer near the firstregion is less than the height near the second region; performing afirst implantation by using a first type of dopant of a secondconductivity type on the first region of the semiconductor layer to forma well of the second conductivity type in the first region of thesemiconductor layer, the first implantation being performed by using afirst energy of implantation; performing a second implantation by usinga second type of dopant of the first conductivity type to form a sourceregion and a drain region of the LDMOS transistor, wherein the sourceregion is formed in the first region and the drain region is formed inthe second region, and wherein the source region is partially in thewell of the second conductivity type and partially underneath theasymmetric conductive spacer; and performing a third implantation byusing a third type of dopant of the second conductivity type on theasymmetric conductive spacer to form a channel region of the LDMOStransistor in the semiconductor layer, the channel region being formedunder the asymmetric conductive spacer near the first region and beingformed essentially completely under the asymmetric conductive spacer,wherein the third implantation is performed by using a second energy ofimplantation and the channel region has a depth that is greatest nearthe first region and decreases away from the first region.
 2. The methodof claim 1, wherein a drift region of the first conductivity type isformed between the channel region and the drain region when a gatevoltage is applied to the LDMOS device.
 3. The method of claim 1 furthercomprising: forming the dielectric layer on the semiconductor layerafter the semiconductor layer of the first conductivity type is formed,wherein the dielectric layer is formed before the asymmetric conductivespacer is formed; and etching the dielectric layer after the asymmetricconductive spacer is formed, wherein etching is performed to remove thedielectric layer in the first region and the second region.
 4. Themethod of claim 3 further comprising: forming a field oxide layer on thesemiconductor layer after the steps of forming the asymmetric conductivespacer and etching the dielectric layer are performed; and etching thefield oxide layer to define an area of the semiconductor layer where theLDMOS transistor is to be fabricated, wherein etching of the field oxidelayer is performed such that the field oxide layer surrounds the area ofthe semiconductor layer.
 5. The method of claim 1, wherein thesemiconductor layer is an epitaxial layer of the first conductivitytype.
 6. The method of claim 1, wherein the first conductivity type isN-type and the second conductivity type is P-type.
 7. The method ofclaim 1, wherein the first type of dopant and the second type of dopantare selected from the group consisting of boron, gallium and indium, andthe third type of dopant is selected from the group consisting ofarsenic and phosphorus.